IP addressing
IP addressing
When TCP/IP is implemented on an internal network that connects to the Internet - whether through a dial-up or dedicated connection - care must be taken to choose IP addresses that will not conflict with addresses already in use on the Internet. Configuring a network interface card for TCP/IP will not guarantee that valid IP addresses will be assigned. IP address assignment must be a part of the overall network planning process that takes place before the network is implemented. Two options are available for doing this:
- Use registered IP addresses available from your ISP or InterNIC
- Use name translation devices such as firewalls or proxy servers in conjunction with IP addresses reserved for use on private networks. InterNIC has guaranteed that the reserved addresses will never be used on the public Internet
Made-up addresses that do not meet the above criteria can be used on private networks that will never be connected to the InterNet. But for networks that are currently connected or that may be connected to the Net at some date in the future, using appropriate IP addresses is critical. Failure to do so may cause connection instabilities or may prevent you from connecting to the Internet at all.
Every IP address can be broken down into 2 parts, the Network ID (netid) and the Host ID (hostid). All hosts on the same network must have the same netid. Each of these hosts must have a hostid that is unique in relation to the netid. IP addresses are divided into 8 octets with each having a maximum value of 255. We view IP addresses in decimal notation such as 196.15.01.121, but it is actually utilized as binary data.
| DECIMAL | BINARY | |
| 128 | 10000000 | |
| 64 | 01000000 | |
| 32 | 00100000 | |
| 16 | 00010000 | |
| 8 | 00001000 | |
| 4 | 00000100 | |
| 2 | 00000010 | |
| 1 | 00000001 |
CLASSES
IP addresses can be class A, B or C. Class A addresses are for networks with a large number of hosts. The first octet is the netid and the 3 remaining octets are the hostid. Class B addresses are used in medium to large networks with the first 2 octets making up the netid and the remaining 2 are the hostid. Class C is for small networks with the first 3 octets making up the netid and the last octet the hostid.
| CLASS | RANGE | |
| A | 10000000 | |
| B | 01000000 | |
| C | 00100000 |
SUBNET MASK
A subnet mask blocks out a portion of an IP address and is used to differentiate between the hostid and the netid. The part of the IP address blocked out by 255 is the netid. The default subnet masks are as follows:
| CLASS | DEFAULT SUBNET | # OF SUBNETS | # OF HOSTS PER SUBNET | |||
| Class A | 255.0.0.0 | 126 | 16,777,214 | |||
| Class B | 255.255.0.0 | 16,384 | 65,534 | |||
| Class C | 255.255.255.0 | 2,097,152 | 254 |
The following table shows a comparison between the different subnet masks.
| MASK | # OF SUBNETS | CLASS A HOSTS | CLASS B HOSTS | CLASS C HOSTS | ||||
| 192 | 2 | 4,194,302 | 16,382 | 62 | ||||
| 224 | 6 | 2,097,150 | 8,190 | 30 | ||||
| 240 | 14 | 1,048,574 | 4,094 | 14 | ||||
| 248 | 30 | 524,286 | 2,046 | 6 | ||||
| 252 | 62 | 262,142 | 1,022 | 2 | ||||
| 254 | 126 | 131,070 | 510 | Invalid | ||||
| 255 | 254 | 65,534 | 254 | Invalid |
IP adressing resources
- http://www.3com.com/nsc/501302.html
- http://support.wrq.com/tutorials/tcpip/ipadd1.htm
- ftp://sunsite.unc.edu/pub/linux/docs/HOWTO/IP-Masquerade-HOWTO
